1. Field of the Invention
This invention relates to improvements in a process for the preparation of polymer compositions having low volatile content by the continuous removal of volatile components, such as unreacted monomers, solvents, chain transfer agents and low-molecular-weight polymers, from a polymerization fluid composition containing a styrene polymer.
2. Description of the Prior Art
In the prior art relating to the continuous removal of volatile components from a polymerization fluid composition, there are a number of well-known methods which involve using a multitubular heat exchanger to preheat, with or without foaming, a polymerization fluid composition containing volatile components and then flashing it into a vacuum vessel (see, for example, Japanese Patent Publication Nos. 31678/'70 and 29797/'73 as well as Japanese Patent Laid-Open No. 134781/'76).
In these prior art methods, the effective means for reducing the volatile content of the resulting polymer composition as much as possible are to heat the polymerization fluid composition containing volatile components to higher temperatures by means of the preheater and to enhance the degree of vacuum of the vacuum vessel. However, as the temperature of the polymerization fluid composition is raised, especially the residual monomer or monomers polymerize in the preheater and, hence, the content of low-molecular-weight polymers in the resulting polymer composition increases. Thus, excessively high temperatures of the preheated polymerization fluid composition have the disadvantages of reducing the thermal resistance of molded articles, causing oily matter to adhere to the mold during molding operation, and bringing about an unfavorable color change and, in some cases, an impairment of transparency depending on the heat history of the polymer composition. On the other hand, enhancement of the degree of vacuum of the vacuum vessel causes a marked increase in the gaseous volume flow rate of volatile components, resulting in an increased pressure loss within the piping and condenser. Such equipment requires immense construction costs. Moreover, where the amount of volatile components to be separated by evaporation is large, it is practically impossible to construct equipment exceeding a certain degree of vacuum.
Thus, the above-described means have their limits, so that it has been extremely difficult to reduce the content of the volatile components (such as unreacted monomers, solvents, chain transfer agents and low-molecular-weight polymers) remaining in the polymer composition to a sufficiently low level. For this reason, the additional use of a vented extruder, a thin-film evaporator or the like has been proposed for the purpose of reducing the volatile components remaining in the product polymer to the fullest extent. However, these methods cause an increase in equipment and power costs, resulting in increased production costs. Thus, it is highly desirable to overcome the above-described disadvantages.